There are some studies in which the dynamics of a helicopter carrying an external payload are considered. However, the vast bulk of these studies are confined to simulating and analyzing the system without making any suggestions on how it can be stabilized. The first time any approaches can be found to solving the problem of vertical oscillations is in Gabel and Wilson (Test Approaches to External Sling Load Instabilities, 24th Annual National Forum, AHS-230, American Helicopter Society, 1968).
The problem of vertical oscillations affecting the CH-53G transport helicopter used by Germany's federal armed forces was studied by the German Federal Office of Defense Technology and Procurement back in the early 80's (see Buchacker, E., Flight Test Evaluation of the PIO-behaviour of a Helicopter with Suspended Cargo, AGARD, CP333, Paper 24, June 1982).
A simplified explanation of the mechanism by which this effect arises can be given on the basis of the mechanical equivalent model, consisting of three masses connected by springs and dampers, which is shown in FIG. 2.
What can be considered critical is a coincidence of the resonant frequency with one of the frequencies of structures which occur in the cockpit of the helicopter. By a feedback coupling of the oscillation through the pilot's body and arm to the collective control (a biomechanical coupling), it is possible that the system is excited in such a way that an unstable oscillation arises. An oscillatory phenomenon of this kind is referred to as pilot assisted oscillations (PAO's). The aim of currently proposed solutions to this problem is to find a way around this system-related resonance. In the literature, a distinction is made between the following approaches to a solution:
Prevention of Resonance:
What is meant by “prevention of resonance” is the deliberate selection of the lifting gear in the light of the resonant frequency of the vertical oscillation. The intention of this approach is to make the resonant frequency of the vertical oscillation lower than the frequency of any relevant excitation. To this end, it is essential that appropriate information exists on the lifting gear. It is also considered to deliberately provide the lifting gear with particular elastic properties.
Isolating/Damping Member:
In this approach, a further elastic element is inserted between the helicopter and the lifting gear which, because of the connection in series, ensures that the resonant frequency will not exceed a fixed maximum value and that it is thus always sufficiently far away from the frequency of any relevant excitation.
Modification of the Collective Control:
The transmission characteristics of the control are altered by making modifications to the control system of a helicopter carrying a suspended payload such that high-frequency inputs of low amplitude are suppressed at the output but normal control inputs which are of flight mechanical relevance are transmitted unaltered. A simple modification to achieve this result is to increase the friction at the collective control. However, the increase which is thereby obtained in the amount of lag at the control also involves a reduction in stability. Releasing the collective control if severe oscillations occur has a similar aim. However, this is merely a reaction to a problem which has already occurred and, what is more, is not something which can be safely done in every situation in flight.
Particularly when external payloads are transported with the German federal defense forces' CH-53G helicopter, the only ways in which attempts have so far been made to stop oscillations are by releasing the control or by increasing the friction at the collective control (with a friction brake). If a critical flight condition caused by coupling between a helicopter and its external load cannot be obviated by action of the pilot, the external load has to be dumped. As a result, the helicopter at once becomes controllable again. However, the application of this procedure calls for the pilot to be trained to a high level and to have had extensive experience in the field of flying with external loads.
The object of an arrangement for isolating/damping oscillations is to suppress the transmission of forces and oscillatory energy to the object which is to be isolated. There are known systems which effect damping or isolation either passively or adaptively.
Passive systems work entirely without any energy input or signal processing. The system components formed by spring and/or damper elements are merely able to store or dissipate energy. The only way of affecting the dynamic behavior of the system is by changing components. Passive systems also include systems where the points to which the system to be isolated is connected are the natural nodes of the oscillation of the exciting system. This method is used for example for the insulation of helicopter airframes against vibration. However, with systems of this kind it is possible to suppress very specific frequencies or narrow bands of frequencies, only. Hence they are only suitable for use if the excitation takes place repeatedly at the same defined frequency (e.g. if the working point or operating point is fixed), and if the excited system is not subject to any variations (different distributions of the mass etc.) in operation. There is also the danger that passive systems may not dissipate enough energy for the particular requirements.
In a particular known method of and a device for damping vertical oscillations in a helicopter carrying a suspended external payload a passive damper which is arranged between the load attachment point and the lifting gear in parallel to a load-carrying spring applies forces which are opposed to the vertical oscillations which act at that point.
In adaptive systems, the properties of the passive elements are altered by a controlling action—generally manual. The energy consumption is confined to that required for the adjustment of the properties of the elements. As compared to the exciting frequency, the frequency with which adaptive action is taken is very low and thus inflexible.
A continuous variation in the properties of the passive elements of the system by means of a control loop is characteristic of a semi-active system. There is no exchange of energy with the oscillating system. The energy consumption comprises merely that used for the adjustment and for signal processing.
What all the above-mentioned approaches have in common is that they do not reduce the susceptibility of the system to vertical oscillations but are intended simply to prevent critical excitations from occurring; thus, there is no increase in the damping of critical resonant oscillations, but a simple shift of the resonant frequency away from the frequency of the relevant excitations occurring. There is thus still a potential risk of resonant oscillations being excited. Further, these systems can only be designed to deal with a limited number of discrete excitation frequencies or a small range of excitation frequencies, and they call for a great deal of training in and experience of flying with external payloads.
However, a characteristic of the actual transportation of external payloads is a large number of variations in configuration (e.g. of the helicopter and the weights of the loads, of the lifting gear used, of the pilot's control strategy, etc.). These variations shift the resonant frequencies of the overall system, which are dependent above all on the distribution of the masses. Hence, systems for preventing resonance at discrete frequencies are not capable of general use with strongly varying mass distributions and elasticities.
Active damping systems which actively generate forces opposed to oscillations in order to suppress oscillations or vibrations in or of a structure are basically known. However, the known principles cannot readily be applied to the damping of vertical oscillations in a helicopter carrying a suspended external load. What is missing for this purpose is, in the first place, a fixed reference system. In flight, a helicopter is suspended from its rotor, i.e. a free-floating and damped generator of thrust, and not from any fixed point. Further, when the payloads transported frequently change and the lifting gear used changes likewise, it would be extremely costly and complicated to obtain information on how the external loads will oscillate.
The description of the prior art in published German patent application DE 10 2005 022 212 A1 mentions an active method of and an active device for damping vertical oscillations in a helicopter carrying a suspended external payload, in which an active damping member is connected in series with a cable by which a load is suspended from a helicopter to reduce oscillations in the direction of the distance between the helicopter and the suspended load. However, the difficulty of obtaining an input signal for this purpose is pointed out, and the invention which is described in this patent application is not an active damping system but a method of indicating the state of movement of the load suspended from the helicopter to cause the pilot of the helicopter to take damping actions intuitively.
European Patent EP 0 425 352 B1 also published as DE 690 15 572 T2 discloses a device for the active damping of oscillations and mentions the possibility of frequency selective analyzing the signals from oscillation sensors by looking at a fundamental frequency.
From European Patent EP 0 740 746 B1 also published as DE 694 30 264 T2 and corresponding to U.S. Pat. Nos. 5,435,531 and 5,704,596 it is known for a vibration measuring arrangement, which has six accelerometers for measuring vibrations in all six degrees of freedom, to be arranged in the passenger compartment of a helicopter to enable active damping of the vibrations for providing a vibration-free flight to the passengers in the helicopter.
A need remains for a method of and a device for damping vertical oscillations in a helicopter carrying a suspended external payload which, with a limited amount of equipment and a limited expenditure of energy, are capable of applying a high degree of damping to relevant vertical oscillations without being affected by the variations in configuration which occur when different payloads are being transported.
U.S. Pat. No. 3,904,156 discloses an external load stabilisation apparatus improving modal damping of pendular oscillations of an external payload.